1. Field of the Invention
The present invention relates to printing color documents, and more particularly to performing color matching when printing color documents. Still more particularly, the present invention relates to a method for object based color matching when printing color documents.
2. Description of the Prior Art
As color input, display and output devices for computer systems proliferate, the need for color management increases. This is due to several factors. First, differing device types operate in different color spaces. For example, color monitors typically display colors as combinations of red, green, and blue, and are said to work in the RGB color space. Printers typically print images as combinations of cyan, magenta, yellow and black, and are said to work the CMYK color space.
The way colors are sampled in different devices is another factor. Generally, the method used by monitors and scanners to capture color follows the laws of additive color mixture. Additive color mixture adds the color together to yield the result. Additive color mixture moves a color toward white, and usually results in vivid images. Printers, however, typically follow the laws of subtractive color mixture. In subtractive color mixture, color data specifies how much of a certain color to remove from white to yield the result. Consequently, a subtractive color mixture moves colors toward black or dark gray.
Furthermore, different color devices have different color capabilities. Every color device, such as a scanner, printer, or monitor, has a range of colors that it can produce. This range of produceable colors is known as a gamut. Those skilled in the art will recognize that color monitors can produce and display hundreds to thousands of colors. Color printers also produce a range of colors. However, printer colors typically do not coincide with the colors on a monitor. Consequently, in most situations, the gamut for a color monitor only partially overlaps the gamut for a color printer. As a result, some colors displayed on a color monitor can not be produced by a color printer.
Finally, devices of the same type that are manufactured by different manufacturers may produce different colors, or intensities of the same colors, for the same color data. For example, color monitors made by different manufacturers may display different colors, or intensities of the same colors, for the same RGB values. Print technologies vary drastically, and the gamut that an ink jet color printer can print may be quite different from a printer based on a different technology, such as a color laser printer. A single printer may have a fluctuating gamut depending on the paper or ink being used at the time of printing.
Color matching typically means converting colors between differing gamuts. Most current color management systems match colors pursuant to a rendering intent. A rendering intent affects the way colors are matched by causing the colors to be matched with a particular use in mind. For example, a document that will be used for business purposes is generally printed with saturated colors. The user is not concerned with matching the printed colors as close as possible to the original colors. Saturated colors when printed are typically vivid colors. This type of rendering intent is known as business/graphics.
Another type of rendering intent is photographic. A document that is, or includes, a photograph may be printed with colors that are matched perceptually. Photographic rendering intent attempts to give the best overall match of colors.
Objects are anything that can be drawn, and include text, rectangle, oval, region, polygon, and PixMaps. Unless each object is individually tagged with a rendering intent, most current color management systems typically match colors in a document using only one rendering intent. They do not allow a user to print individual pages within a document with different rendering intents. They also do not allow colors to be matched on a single page with different rendering intents. This can create problems for documents or pages that contain photographs and text or charts. The user can print the document with a business/graphics rendering intent, but the photographs within the document will typically look dark, and the colors within the printed photographs will not match the original colors very closely. Alternatively, the user can print the document with a photographic rendering intent, but the text and charts within the document will typically print with less saturated colors. These two alternatives are inadequate and unsatisfactory for many users, such as graphic artists or business persons.